This invention relates to a biaxially stretched polyester film suitable for magnetic tape having improved abrasion resistance, slipperiness and reduced occurrence of "drop out".
In general, polyester film, especially polyethylene terephthalate film is used wherever a magnetic, photographic, or electric film is needed. Film used in magnetic tapes, discs and sheets is coated or metallized on one side of the film with magnetizable metal oxide. These coated films are finding an ever-increasing number of applications including audio and video tapes and data storage devices.
The thickness of the coated magnetic layer for a typical video cassette is about 5 .mu.m and the thickness of the metallized magnetic layer is about 1 .mu.m.
However, thin film is generally not very slippery and this prevents the formation of smooth, regular rolls of magnetic tape. Accordingly the base film must be very slippery. Furthermore, roughness on the base film surface damages the thin magnetic layer causing "drop out" or loss of minute portions of the magnetic material itself, causing loss of information. So, the base film must have a smooth surface.
In producing and using magnetic tape, there is considerable physical contact of the film with the equipment. This causes the film to abrade and this forms a white polyethylene terephthalate powder. The useful life of the magnetic tape is significantly decreased. White powder on the surface of the uncoated side of the film is deposited on the coated side covering portions of the coating when the tape is wound. This powder can cause separation of the tape from the reading head, resulting in a loss of information, i.e., "drop out". To remedy this, the base film must have improved abrasion resistance.
It is well known that there are many methods for improving the slipperiness of polyester film. For example, pigments such as SiO.sub.2, TiO.sub.2, clay or bentonite in film is described in U.S. Pat. No. 3,821,156, British Pat. No. 1,359,892, U.S. Pat. No. 3,980,611, U.S. Pat. No. 3,884,870, British Pat. No. 1,306,634 and U.S. Pat. No. 3,221,226. This procedure is hereinafter referred to as the "pigment method". Another method for forming particles from a catalyst or stabilizer during the polymerization process is described in U.S. Pat. No. 2,921,051, U.S. Pat. No. 3,048,564 and British Pat. No. 1,398,178 and is hereinafter referred to as the "polyester particles method". Explaining the latter method in more detail: residues of a compound such as a catalyst, stabilizer or additive react with polyester components such as monomers or oligomers. The product of this reaction forms particles which are insoluble in the polymer.
A polyester film with good slipperiness and reduced vulnerability to "drop out" for use as video tape, should have the following characteristics. The maximum surface roughness should be in the range of from 0.02 to 0.5 .mu.m, preferably from 0.05 to 0.35 .mu.m, and the density of surface protrusions should be at least 20 surface protrusions per mm. Especially rough protrusions may damage the thin coating so the number of rough protrusions having a height of 0.35 to 0.5 .mu.m should be no more than ten protrusions per mm.sup.2.
However, the inclusion of pigments in amounts large enough to produce asperities on the film surface required for improved slipperiness is often accompanied by agglomeration of pigments and increasing occurrences of "drop out". The inclusion of pigments to improve slipperiness does essentially nothing to improve the abrasion resistance.
On the other hand, the tendency of polyester particles to agglomerate is somewhat less than that of pigments in general. However, well known polyester particles formed from alkaline earth metals or their compounds are accompanied by numerous rough protrusions with heights of at least 0.35 .mu.m and thus increases the occurrence of "drop out". Furthermore, we have tried to produce a film with smaller polyester particles containing lithium alone or in combination with calcium and phosphorus, but such particles failed to improve the abrasion resistance of the film. Also, the inclusion of both pigments and polyester particles containing calcium and phosphorus has been known in the art and is described in Japan Kokai 51-81811. Here also there is no improvement in the abrasion resistance or reduction in the occurrence of "drop out".
It is an object of the present invention to provide an improved base film for magnetic tape. The magnetic tape made from film of this invention has excellent winding, slipping and moving characteristics, reduction in occurrence of "drop out", and outstanding resistance to abrasion. Another object of the present invention is to provide a polyester film which is especially suitable for video tape. A further object of the invention is to provide a polyester film which is suitable for audio tape.
A film according to this invention is a biaxially oriented polyester film for magnetic tape consisting essentially of ethylene terephthalate units, said film including polyester particles in an amount from about 0.05% to 1.0%, by weight, said particles having an average size of about 0.10 to 3.0 .mu.m and containing about 0.03% to 5.0%, by weight, lithium, about 0.03% to 5.0%, by weight, calcium and about 0.03% to 10.0%, by weight, phosphorus. Said film also includes in an amount of about 0.001% to 0.7%, by weight, inert pigments, said pigments being selected from the group consisting of oxides or salts of elements in Groups II, III or IV of the periodic table and having an average size of about 1.0 to 10.0 .mu.m. Other and further objects, features and advantages of this invention will become more fully apparent from the following detailed description. The term "average size" is defined as the diameter of the sphere which most closely approximates the volume of the particle in the film.